1. Field of the Invention
This invention relates to an input processing circuit for an air flow sensor for use in measuring a suction air flow rate of an internal combustion engine, for example, a hot wire type air flow sensor.
2. Description of the Prior Art
A hot wire type air flow sensor is one type of air flow sensor employed for measuring a suction air flow rate of an internal combustion engine. This air flow sensor is constructed such that a voltage corresponding to an air flow rate passing within the sensor is outputted. An input processing circuit for the air flow sensor processes the above-mentioned output voltage to transmit a result of processing operation to an analogue value-digital value converting IC (Integrated Circuit). With this arrangement, a fuel injection device storing the input processing circuit therein judges an air flow rate sucked by an internal combustion engine and injects an amount of fuel corresponding to the air flow rate.
FIG. 1 is a circuit diagram for showing an input processing circuit for a conventional type of air flow sensor. In this figure, a reference numeral 1 designates a hot wire type air flow sensor, and a reference numeral 2 denotes a fuel injection device having an input processing circuit 2c therein.
A reference numeral 1a within the air flow sensor 1 indicates an amplifier, wherein an output voltage V.sub.AFS is outputted from an output terminal end of the amplifier 1a to a terminal end (a) of the air flow sensor 1 and a reference voltage V.sub.A is applied to a terminal (b).
The terminal ends (a) and (b) are connected to terminal ends a1 and b1, respectively, of a fuel injection device 2 through lines 3a and 3b, respectively. The terminal a1 is connected to the ground through resistors R1 and R2 and at the same time a connection point between resistors R1 and R2 is connected to a non-inverting input terminal end [(+) input terminal end] of an operational amplifier 2a. The terminal b1 is connected to an inverting input terminal end [(-) input terminal end] of the operational amplifier 2a through resistor R3.
Between an output end of the operational amplifier 2a and the inverting input end is connected a resistor R4. The output end of the operational amplifier 2a is connected to an output terminal 2b.
This input processing circuit 2c is operated such that it may process an output voltage of an air flow sensor 1 and transmit it to an analogue value - digital value conversion IC (not shown). The operational amplifier 2a is a circuit for performing the above-mentioned processing.
Operation of the circuit shown in FIG. 1 will be described. To the terminal end (a) of the hot wire type air flow sensor 1 is outputted a voltage V.sub.AFS corresponding to a flow rate of sucked air in an internal combustion engine. That is, the amplifier 1a may output the voltage V.sub.AFS in reference to the reference voltage V.sub.A.
The input processing circuit 2c stored in the fuel injection device 2 compares a voltage V.sub.IN at the terminal a1 as an input voltage with a voltage V.sub.B at the terminal b1 as its reference voltage through the operational amplifier 2a, generates an output voltage V.sub.O in reference to an equation of ##EQU1## and then transmits it to the analogue value - digital value conversion IC.
When noises caused by a high voltage application of a spark plug of an internal combustion engine or noises caused by a turning-on or turning-off of a relay in other mounted systems induce voltages between the terminal ends (a) and a1 or terminal ends (b) and b1 shown in FIG. 1, the output voltage V.sub.O of the operational amplifier 2a of the input processing circuit 2c becomes ##EQU2## where, a noise level between the terminal ends (a) and a1 is V.sub.N1 and a noise level between the terminal ends (b) and b1 is V.sub.N2, and therefore an error voltage V.sub.ERR becomes ##EQU3## and so it shows a certain problem to cause such an error as indicated above equation.
FIG. 2 is a circuit diagram for showing an input processing circuit for another conventional type of air flow sensor. In this FIG. 2, a reference numeral 1 designates a hot wire type air flow sensor and a reference numeral 1a denotes an amplifier for generating an output voltage of the air flow sensor.
A reference volta V.sub.A is applied to the amplifier 1a to cause a consumption current I.sub.1 for operating its circuit to flow to the ground and further an output voltage V.sub.AFS to be outputted.
An input processing circuit 2c is stored in the fuel injection device 2. This input processing circuit 2c mainly consists of an operational amplifier 2a. To a non-inverting input terminal end [(+) input terminal end] of the operational amplifier 2a is applied an output voltage V.sub.AFS of the amplifier 1a as a voltage V.sub.IN.
The operational amplifier 2a may amplify a differential voltage of the voltage V.sub.IN in reference to the voltage V.sub.B. An inverting input terminal end [(-) input terminal end] of this operational amplifier 2a is connected to the ground through a resistor R.sub.12 and at the same time a resistor R.sub.11 is connected between the output terminal end of the operational amplifier and an inverting input terminal end. An output voltage V.sub.O is generated for an output terminal end 2b from the output terminal end of the operational amplifier 2a.
A reference numeral 3 designates a battery for the air flow sensor 1 or the input processing circuit 2c.
Operation of the circuit shown in FIG. 2 will be described. In the hot wire type air flow sensor 1, an output voltage V.sub.AFS corresponding to a suction air flow rate of an internal combustion engine is outputted by the amplifier 1a in reference to the reference voltage V.sub.A.
In the input processing circuit 2c stored in the fuel injection device 2, the operational amplifier 2a causes a voltage V.sub.IN to be applied as an input voltage in reference to the voltage V.sub.B across both ends of a resistor R.sub.12, generates an output voltage V.sub.O in reference to an equation ##EQU4## and then transmits it to an analogue value - digital value conversion IC.
In the hot wire type air flow sensor 1, a ground potential acting as a reference value of the output voltage V.sub.AFS of the amplifier 1a is increased up to a value of V.sub.1 by the consumption current I.sub.1 for use in operating the circuit.
In turn, also in the input processing circuit 2c, a ground potential of the input processing circuit 2c is increased up to V.sub.2 due to a consumption current I.sub.2 of the input processing circuit 2c in another circuit in the fuel injection device 2.
Further, since an electric current in a circuit including another installed equipment flows toward the battery 3, a potential V.sub.12 is also generated between a ground point of the air flow sensor 1 and a ground point of the fuel injection device 2.
Therefore, a voltage to be inputted to the input processing circuit 2c becomes EQU V.sub.IN =V.sub.AFS -[(V.sub.2 +V.sub.12)-V.sub.1 ] (5)
in respect to an output voltage V.sub.AFS of the amplifier 1a to have an input error of [(V.sub.2 +V.sub.12)-V.sub.1 ]. This error is transmitted to an analogue value - digital value conversion IC and further the fuel injection device 2 shows an error of fuel injection volume corresponding to this input error.